The present invention relates to dental appliances and structural components thereof. More particularly, it is concerned with passive or nonforce-imparting dental appliances and with dental systems using passive components made from fiber-reinforced composite plastic materials.
Dental appliances are employed for a wide range of purposes, but generally can be classified into force-imparting and nonforce-imparting applications. The former are generally associated with orthodontic appliances where forces are used to move teeth and bone to more desirable positions. Our prior developments of orthodontic systems using beta titanium metallic alloys and fiber-reinforced composites in orthodontic systems are reported, respectively, in U.S. Pat. No. 4,197,643 and in copending U.S. Patent Application Ser. No. 817,925, now U.S. Pat. No. 4,717,341.
Nonforce-imparting or passive dental appliances are intended to restore or replace teeth, to retain natural teeth in a desired position subsequent to orthodontic treatment or tooth loss, to prevent migration and to give added support to a weakened periodontium. These appliances often include structural components which function as bars, wires, beams, posts, clasps and other complex shapes, etc. For example, retainers typically employ a wire partially contoured to the teeth to maintain their positions. The wires may be imbedded in a plastic portion which, along with embedded wire clasps, retain the appliance in position. Periodontal splints employ a wire bonded to the teeth to stabilize them following periodontal surgery or for other reasons. Children often are fitted with a space maintainer, which is an appliance that incorporates beams attached to the teeth on either side of an edentulous site or connected to molars on either side of the mouth. Removable partial dentures employed heretofore, are metallic frameworks to which prosthetic teeth are fastened. Fixed prosthetic appliances, including traditional bridges and acid-etched bridges, incorporate bars and beams for structural integrity. Other passive dental appliances also incorporate structural parts.
The devices described above and other passive dental appliances could be divided into the following five categories of appliances: fixed, removable, fixed-removable, tooth replacement, and passive orthodontic and temporo-mandibular joint (TMJ), appliances. Fixed appliances or retainers are bonded directly to the teeth and are used to retain teeth after orthodontic treatment or to serve as periodontal splints. Removable appliances or retainers are not directly bonded to the teeth but instead are carried or supported by soft tissue and teeth. Removable appliances are used for retention and tooth replacement. Fixed-removable appliances or retainers also are carried or supported by soft tissue and teeth but, in addition, they are connected to fixed attachments that are separately bonded or attached to the teeth. Fixed tooth replacement appliances are used to replace missing teeth, temporarily or permanently, or to act as space maintainers. The materials and techniques described herein also can be used to restore individual missing teeth with a band, crown, veneer, or smaller restoration. Finally, passive orthodontic and temporo-mandibular joint appliances transmit or use muscle forces to alter growth, to guide dental development, and to alter TMJ and muscle functions or habits. Examples include functional appliances, headgears, bite opening and disc recapturing appliances and lip bumpers. Although these five categories of appliances are by themselves passive and nonforce-imparting, they can be used in combination with force imparting appliances as separate or integral units.
Traditionally, the types of appliances described have been constructed from either metal alloys or polymers. Alloys such as chromium-cobalt-nickel, Cr-Co-Ni, are commonly used in partial denture frameworks. Fixed prosthodontic devices employ gold-based, palladium-based and Cr-Co-Ni alloys. Retainers and periodontal splints incorporate stainless steel wires or bars while space maintainers us stainless steel wires. The polymer used in virtually all dental structural appliances, whether it be a component, as in a retainer, or the entire appliance, as with a complete denture, is an acrylic polymer such as polymethylmethacrylate (PMMA).
There are various characteristics which are important for any nonforce-imparting dental structural component. Probably the most important of these is high stiffness and strength with minimum bulk. The structural component is often maintaining or retaining the relative positions of dental tissues or acting as part of a prosthetic appliance. In each case, the component is expected not to deflect or deform under stress. Therefore high rigidity or stiffness is important. Appliance stiffness is dependent on the moment of inertia of the geometric shape of the device and the modulus of elasticity of the material. To a limited practical degree bulk and modulus can be interchanged to maintain stiffness. For example, a fixed prosthetic appliance made from a Cr-Co-Ni alloy can be slightly thinner than one made from a gold-based alloy because the Cr-Co-Ni alloy has a greater modulus of elasticity. In addition to stiffness and strength, other important characteristics for a nonforce-imparting dental component include ease of processing and appliance fabrication, accuracy of fit, esthetics, ease of joining to itself and to other materials, ease of bonding to hard dental tissue, biocompatibility and structural stability.
Currently no single dental material or appliance design completely satisfies all of these needs. In general, the metal alloys have superior mechanical properties, but require special laboratory processing, are difficult for the clinician to form and adjust and are not aesthetic. Some alloys contain potentially non-biocompatible elements such as nickel and beryllium. The structural polymeric dental materials such as PMMA are easier to process, although the procedures still are usually performed in a special dental laboratory. More significantly, the stiffness and strength of the polymeric materials are low, and require compensating bulk. Some appliances, like retainers, employ both metal and plastic to take advantage of the favorable features of each.
The dental literature contains reports of attempts at improving the mechanical properties of the acrylic polymers by reinforcement with fillers. However, successful applications have been with restorative filling materials that incorporate a particulate filler in a methacrylate matrix such as bisphenol-A glycidyl methacrylate (BIS-GMA) or polyurethane dimethacrylate. These particular filled plastics are not fiber-reinforced and are used as filling materials, cements or veneers, but not as structural components of the type described herein.
Reported attempts of using fiber reinforcement for structural components have been unsuccessful. Two recent reports have been by R. H. Mullarky, Journal of Clinical Orthodontics Vol 19, No. 9 p 655-8, Sept. 1985 and by DeBoer, et. al, Journal of Prosthetic Dentistry Vol 51, No 1 p. 119-121 Jan. 1984. In both cases, the authors attempted to place fibers into traditional dental acrylic, while simultaneously forming the desired appliance. While both articles report some improved properties with reinforcement, the degree of improvement is much less than would be expected from an effective fiber reinforced composite. In addition, the techniques are performed by hand thereby possibly causing contamination of the fiber surface and a likely deterioration in the wetting characteristics at the resin-fiber interface. Splints reported by M. F. Levenson, CDS Review pages 23-25, September, 1986, appear to have similar deficiencies. Finally, the techniques are difficult to carry out and are not reliable. DeBoer states that the technical difficulty associated with incorporating the fibers "may outweigh any potential advantage."
It is believed the reason for the earlier lack of success in achieving the expected mechanical properties is that effective composite materials were never formed. The approaches are appealing because the composite and the appliance were formed simultaneously and traditional dental materials and techniques were used. However, the data shows that the strength and stiffness or modulus of elasticity were below expectations. It is quite likely that limited fiber loading coupled with either poor wetting and/or the presence of voids at the resin-fiber interface could account for the poor results.
Grave, Chandler and Wolfaardt, Dent. Mater. Vol 1, p 185-187 (1985) attempted to reinforce dental acrylics with loosely woven carbon or Kevlar fiber mat. Their results with bending tests of standard test specimens showed a modest improvement in properties for one dental acrylic, and a decrease in strength and stiffness for a second.
The present invention obviates these technical difficulties and provides nonforce-imparting or passive dental components and appliances that possess a preferred combination of properties compared to polymeric compositions used herebefore. This includes not only greater stiffness and strength but also generally higher mechanical properties than those exhibited by the commonly used dental polymers. They are more esthetic and easier to process, form and adjust than dental metallic alloys, thereby allowing for superior and unique designs. Included in this object is the provision for a passive dental component or appliance made from a preformed effective fiber-reinforced composite material via an improved two step process.
Another object of the present invention is to provide passive dental components and appliances of the type described using a two step technique that optimizes the wetting of the fibers while obviating the presence of voids, thereby enabling the incorporation of significantly higher amounts of fiber with concomitant increases in strength and other desired mechanical properties. Included in this object is the provision for the initial production of an effective composite material and the subsequent formation of the dental component from that material.
Still another object of the present invention is to provide passive dental appliances of the type described having improved fiber distribution and dispersion in a fiber-reinforced composite material while avoiding the contamination and deterioration of the fiber-resin interface heretofore associated with hand placement of the fibers. Included in this object is the provision for the effective use of a wide range of resins in the dental components as well as a variety of processing techniques.
A further object of the present invention is the provision for fiber reinforced composite passive appliances such as fixed retainers that can be bonded directly to the teeth and other components; are easily and directly formed into complex shapes requiring minimum skill and simple procedures yet are more intricate, give greater control over tooth position, have lower appliance bulk with comfort, are esthetically pleasing and are not dependent on patient cooperation.
Using the present invention, removable appliances have greater strength with less bulk, allowing superior resistance to fracture and deformation. Their smaller size facilitates patient acceptance and cooperation, less palatal coverage that allows more normal taste during mastication and less complex retention mechanisms. Additionally, the fixed tooth replacement appliances are stronger than existing polymer bridges, require simple direct and laboratory procedures, are relatively non-invasive to adjacent teeth and are esthetically pleasing.
Passive appliances employing the present invention, may have complex curvatures which are more functional and comfortable, are simple to fabricate and most importantly are stronger and more durable. For example, for headgear the invention allows for more complex configurations that are stronger and more efficient. These appliances also can have less bulk, possess better aesthetics and are more comfortable for the patient.
Other objects and advantages will be in part obvious and in part pointed out more in detail hereinafter.
These and related objects are achieved in accordance with the present invention by providing a passive dental appliance or structural component thereof fabricated from an effective fiber reinforced composite material comprised essentially of a polymeric matrix and a fiber component embedded within the matrix. The fiber component of the composite material constitutes greater than 20 percent by weight of the composite material and the modulus elasticity of the composite is greater than 0.5.times.10.sup.6 psi. The beneficial results of the present invention are achieved by utilizing a system that employs a two-step procedure. First, an effective fiber-reinforced composite material is produced having the requisite stiffness and strength characteristics, fiber orientation and void free wetting of the fibers. .The effective composite material is thereafter formed into the dental device: to achieve improved aesthetics coupled with ease of processing and structural stability. This two-step process allows for the optimum development of the most advantageous mechanical properties.
A better understanding of the invention will be obtained from the following detailed description of the illustrative applications of the invention including the several components of the invention and the relation of one or more of such components with respect to each of the others, as well as the features, characteristics, compositions, properties and relation of elements described and exemplified herein.